A couple of script changes after CI integration

- Renamed struct_.py -> structs.py again.

- Removed lfs.csv, instead prefering script specific csv files.

- Added *-diff make rules for quick comparison against a previous
  result, results are now implicitly written on each run.

  For example, `make code` creates lfs.code.csv and prints the summary, which
  can be followed by `make code-diff` to compare changes against the saved
  lfs.code.csv without overwriting.

- Added nargs=? support for -s and -S, now uses a per-result _sort
  attribute to decide sort if fields are unspecified.

scripts/cov.py

@@ -180,6 +180,7 @@ class CovResult(co.namedtuple('CovResult', [
         'calls', 'hits', 'funcs', 'lines', 'branches'])):
     _by = ['file', 'function', 'line']
     _fields = ['calls', 'hits', 'funcs', 'lines', 'branches']
+    _sort = ['funcs', 'lines', 'branches', 'hits', 'calls']
     _types = {
         'calls': Int, 'hits': Int,
         'funcs': Frac, 'lines': Frac, 'branches': Frac}
@@ -372,8 +373,12 @@ def table(Result, results, diff_results=None, *,
             reverse=True)
     if sort:
         for k, reverse in reversed(sort):
-            names.sort(key=lambda n: (getattr(table[n], k),)
-                if getattr(table.get(n), k, None) is not None else (),
+            names.sort(
+                key=lambda n: tuple(
+                    (getattr(table[n], k),)
+                    if getattr(table.get(n), k, None) is not None else ()
+                    for k in ([k] if k else [
+                        k for k in Result._sort if k in fields])),
                 reverse=reverse ^ (not k or k in Result._fields))
 
 
@@ -617,8 +622,10 @@ def main(gcda_paths, *,
     results.sort()
     if sort:
         for k, reverse in reversed(sort):
-            results.sort(key=lambda r: (getattr(r, k),)
-                if getattr(r, k) is not None else (),
+            results.sort(
+                key=lambda r: tuple(
+                    (getattr(r, k),) if getattr(r, k) is not None else ()
+                    for k in ([k] if k else CovResult._sort)),
                 reverse=reverse ^ (not k or k in CovResult._fields))
 
     # write results to CSV
@@ -626,14 +633,15 @@ def main(gcda_paths, *,
         with openio(args['output'], 'w') as f:
             writer = csv.DictWriter(f,
                 (by if by is not None else CovResult._by)
-                + ['cov_'+k for k in CovResult._fields])
+                + ['cov_'+k for k in (
+                    fields if fields is not None else CovResult._fields)])
             writer.writeheader()
             for r in results:
                 writer.writerow(
-                    {k: getattr(r, k)
-                        for k in (by if by is not None else CovResult._by)}
-                    | {'cov_'+k: getattr(r, k)
-                        for k in CovResult._fields})
+                    {k: getattr(r, k) for k in (
+                        by if by is not None else CovResult._by)}
+                    | {'cov_'+k: getattr(r, k) for k in (
+                        fields if fields is not None else CovResult._fields)})
 
     # find previous results?
     if args.get('diff'):
@@ -748,10 +756,12 @@ if __name__ == "__main__":
             namespace.sort.append((value, True if option == '-S' else False))
     parser.add_argument(
         '-s', '--sort',
+        nargs='?',
         action=AppendSort,
         help="Sort by this field.")
     parser.add_argument(
         '-S', '--reverse-sort',
+        nargs='?',
         action=AppendSort,
         help="Sort by this field, but backwards.")
     parser.add_argument(